Ballast tank arrangement for double-hull tanker

ABSTRACT

There is provided a tanker for carrying petroleum products, including crude oil, chemicals or other potentially environmentally harmful materials, having an inner hull and an outer hull and where the void between the inner hull and outer hull at the bottom of the tanker is at or near the minimum spacing required by U.S. and international regulations. The tanker has side tanks for holding ballast, which extend upward from near to or above the ballast water line. The dimensions of each side tank are large enough to hold a sufficient volume of ballast water to keep the tanker in ballast condition meeting U.S. and international regulations for ballast draft. The cross-sectional shape of the side tank is substantially rectangular in the preferred embodiment, although other shapes to most efficiently utilize the space available on the tanker ship also can be used. The tanker also has one or more cargo tanks located inside the inner hull, which tanks also may be substantially rectangular in cross-section and arranged to meet U.S. and international regulations for oil outflow. The void in the bottom of the tanker is not used for cargo or ballast and remains dry.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to ocean going cargo ships and specifically to double-hull tankers carrying crude oil, petroleum products, chemicals, or other liquids harmful to the environment.

[0003] 2. Discussion

[0004] Prior to the 1970's, it was common in the shipping industry to use cargo tanks to hold ballast water when a tanker was in ballast. This practice was commonly used for tankers carrying petroleum products, including crude oil and chemicals, which resulted in the mixing of ballast water with residual cargo still in the cargo tanks after those tanks had been emptied of cargo and nominally cleaned. The ballast water would then be emptied from the cargo tanks, and some residual cargo would be released with it into the sea.

[0005] In the early 1970's, in the United States as well as in other countries, environmental concerns prompted changes in the practices of the petroleum shipping industry. Among the first changes implemented were regulations that prohibited the use of a single tank as both a cargo tank and as a ballast water tank. The result was a requirement that tankers have separate ballast tanks so that ballast could not mix with the residual content of the cargo tanks, except in emergency heavy-weather situations (“storm ballast”). The effect on the design of tankers was minimal since all that was required was the dedication of certain tanks for ballast water only. One design change that did take place was the creation of a three-tank-across arrangement where ballast tanks were typically placed out on the sides of the tanker, as wing tanks, and cargo tanks were aligned down the centerline of the tanker, as well as along the sides as wing tanks.

[0006] Ballast tanks in the segregated tank design typically extended all the way to the bottom of the tanker and were large box-type spaces, which might have, for example, dimensions on the order of 68 feet tall, 33 feet wide and 70 feet long. This configuration allowed for easy inspection of the ballast tanks. Frequent and thorough inspection of ballast tanks is important because of the corrosive nature of the sea water used as ballast and because ballast water is typically loaded into ballast tanks while the tanker is in port, where the water tends to have a high degree of silt, causing the bottom of the ballast tanks to accumulate mud. The common method of inspecting these types of ballast tanks was for inspectors to float in a raft inside the tank, starting while the tank was nearly full, and floating around to inspect the sides of the tank. The water would slowly be pumped out of the tank, gradually exposing more of the sidewalls to the inspectors. This method, called rafting, was easy to implement, and it was thorough since there was access to the entire depth of the tank.

[0007] In response to the Exxon Valdez oil spill of 1989, various governments and regulatory agencies around the world, including in the United States, adopted laws and regulations to try to prevent similarly devastating oil spills. One such regulation is the United States requirement that new tankers calling on U.S. ports have a double hull completely surrounding the sides and bottom of all cargo oil tanks, the purpose of which is to create a secondary containment of crude oil or other environmentally damaging liquid. If the outer hull of the tanker is damaged, no cargo will be released as long as the inner hull is not also penetrated. The area between the inner and outer hulls is termed a Protective Location Space (PLS).

[0008] In tanker design there is a strong desire to utilize all available space on the presumption that doing so will minimize the cost of construction and operation of the tanker.

[0009] The tendency then, when double-hull tankers meeting the above regulations were first designed, was to use the voids between the inner and outer hull of the tanker as ballast water tanks. The ballast tank essentially was formed down the sides of the tanker and across the bottom in a “U” shape. One variant of the “U” shape was made by inserting a bulkhead at the center of the “U” in the bottom of the tanker, thus creating two L-shaped tanks.

[0010] While the initial design of double-hull tankers is simple and intuitive, there are many problems associated with it. One problem is that the deep and narrow shape of the ballast tanks makes inspection, maintenance and repair difficult. Because of the desire to utilize as much space as possible for cargo, the width of the void between the inner and outer hull in double-hull tankers is minimized as much as can be consistent with the requirements of the regulations and the volume of ballast needed. For example, under U.S. and international regulations, the width between the inner and outer hull can be as small as two meters. The ballast tank, therefore, is relatively narrow and deep, extending from the top of the ship all the way down and across the bottom of the tanker.

[0011] Additionally, the surfaces in the ballast tank can be slippery, making inspection even more difficult. As discussed above, since ballast water taken in while a tanker is in port may contain silt, the surfaces of the ballast tank may become coated with mud. The tight space and other structural constraints severely limit the ability to construct inspection aids, such as walkways, to ease the difficulties of inspection. Rafting is not feasible due to the narrow confines of ballast tanks. In addition, the nature of the space blocks all natural light from these spaces and, because it is filled with water when in use, there can be no permanent electrical light fixtures in the ballast space.

[0012] As mentioned, sea water is corrosive, and the inside surface of ballast tanks is commonly coated in order to withstand the corrosive environment. The tight space inside a ballast tank makes it hard to properly apply a protective coating (paint) to all of the surfaces inside the tanks during construction and to maintain such protective coating after the tanker is in service. In addition, there are a considerable number of structural and piping joints inside the ballast tank which are highly susceptible to corrosion. This is compounded by the possibility of inadequate application of protective coating during construction and maintenance and the difficulty of adequately inspecting all of the surfaces and joints to detect the beginning of corrosion.

[0013] Another problem with conventional double-hull tanker designs is the reduction in stability compared to single-hull designs that can occur under certain combinations of ballast and cargo loading. This reduction in stability is partially owing to the large free surface effects, which are cargo or ballast movements that occur in partially loaded tanks as a tanker heels or leans to one side. This movement of the tanker causes the ballast and/or cargo to shift towards the low side of the tanker, thus shifting the tanker's center of gravity away from the centerline. As the tanker heels, at a certain point, depending upon the tanker's weight and the amount of cargo or ballast, the tanker's ability to right itself may be matched by forces acting on the shifted center of gravity, causing the tanker to stabilize and remain leaning at an angle off center, known as the angle of loll. An external force is then required to upright the tanker. There have been a number of incidents in which tankers have become unstable during cargo operations and, while no tanker has capsized, significant angles of loll have been reported with damage to shore facilities, such as hose-connecting arms. Oil-tight centerline bulkheads can be used to reduce the amount of cargo or ballast that actually shifts to the low side of the tanker (although it will still move in that direction), since at least some of the cargo or ballast is prevented from crossing the centerline of the tanker. However, bulkheads spanning the length of the ship add to the cost of construction and, in the case of centerline bulkheads in ballast tanks, create another obstacle to the already problematic inspection, cleaning, and maintenance of ballast tanks.

[0014] A further issue related to conventional double-hull tankers concerns the pumps and piping necessary to empty the ballast tanks. The tight spaces in the ballast tanks make inspection and repair of the piping difficult. In addition, the introduction of centerline bulkheads in the ballast tanks can double the amount of piping, valves, and monitoring/control devices required, thus increasing the likelihood of having to conduct repairs on the piping.

[0015] Attempts have been made to rectify the problems discussed above related to double-hull tanker design and the placement of ballast tanks in the double-hull void, but none of these attempts has done so while incorporating both the double-side and double-bottom construction required by current regulations. One such attempt can be seen in U.S. Pat. No. 5,445,097, to Fujita, et al., which teaches a tanker having a double side hull, wherein the void between the inner and outer hulls is used as a ballast tank. Cargo tanks are positioned in the center of the tanker, and there is a mid-height deck that separates the cargo tank into upper and lower cargo tanks. The tanker does not have a double bottom.

[0016] U.S. Pat. No. 5,335,615, to Bjorkman, discloses a tanker with centerline cargo tanks and split-level side tanks. The transverse widths of the side tanks, as taught, are generally more than 20% of the entire breadth of the tanker. The upper side tanks are ballast tanks, and the lower side tanks are cargo tanks. A cofferdam with a plastic liner is constructed in the lower side tank, and there are risers from the lower side tank and piping between the lower side tank and the center cargo tank. The cofferdam is expected to help prevent sea water from entering the lower side tank in the event the tanker is damaged. If the water does enter the tank, the cargo oil, which floats on the water, is forced up the riser and through the piping, which has a check valve, thus preventing the oil from escaping out of the tanker. A double bottom, required by current regulations, is not taught, however, nor is the use of a lower side tank for cargo consistent with the current regulations.

[0017] There have also been attempts to merely modify the design of conventional double-hull tankers to correct other problems which have been experienced. These, however, do not address the problems identified above. For example, U.S. Pat. Nos. 5,158,031 and 5,275,118 (which is a continuation of the first), both issued to Arnett, et al., teach a conventional double-hull tanker utilizing several different ballast tank configurations. One such configuration involves a ballast tank spanning half the breadth of the double bottom of a tanker and having a watertight enclosure extending from an upper deck to the double bottom level. The problem these inventions were attempting to solve stems from the reduction in stability that double-hull tankers face owing to the higher center of gravity and the effect which this has when the bottom of a tanker is breached in an accident. These patents contend that the use of the above described ballast tank configuration will increase a tanker's stability in the event the bottom is breached.

[0018] Another similar patent is U.S. Pat. No. 5,901,656, to Cheung, which teaches a conventional double-hull tanker with ballast tanks in the double sides and double bottom and with the added inclusion of a mid-level bulkhead which divides the side ballast tanks into an upper wing tank and a lower wing tank. The purpose of the mid-level bulkhead is to relieve the pressure exerted on the double bottom ballast tank and internal bulkheads. When the tank is loaded with ballast, the double bottom and stacked side tanks are intended to be loaded with salt water ballast, similar to the conventional double-hull tanker represented by current art.

[0019] Japanese patent No. 07002165, issued to Tsukudu, et al., teaches two alternative uses for the space between the inner and outer hulls of conventional double-hull tankers. In Alternative I, the entire space between the inner and outer hulls is arranged as a void, i.e., neither salt water ballast nor cargo oil is loaded into these spaces at any time during the ship's life. The distance between the inner and outer hulls is limited to the requirements of regulatory bodies for Protected Location Spaces. Additional segregated ballast tanks are provided for salt water within the inner hull, thereby increasing the size of the ship. This approach meets all regulatory requirements for double hulls, but leaves a narrow double-hull which requires inspection and maintenance while increasing the initial cost of the ship for added salt-water ballast tanks.

[0020] In Alternative II, the double bottom space is treated as a void, while the double sides extend to the bottom of the outer hull and are designed as salt water ballast tanks. The double bottom space does not extend to the full width of the ship. Again, the double side wing tanks remain as narrow spaces meeting the PLS international regulatory criteria, with the remaining required ballast capacity provided by salt-water ballast tanks within the double hull. This arrangement has the same advantages and disadvantages as Alternative I, complicated by reduced access for maintenance and inspection in the double side tanks compared to conventional double-hull designs.

[0021] U.S. Pat. No. 5,353,728, issued to Strange, posits a tank arrangement that is configured without double sides and with cargo tanks divided into pairs, port and starboard, by two longitudinal bulkheads that form the sides of centrally located ballast tanks. All, or a high proportion, of salt water ballast is contained within the central ballast tanks. This arrangement is claimed to offer improved inspection and reduced maintenance while providing equal or better protection from pollution compared to conventional double-hull tankers with PLS ballast spaces between the inner and outer hulls. However, this tank arrangement does not meet current U.S. and international regulations for tanker design and construction.

[0022] Thus, it is desirable to design a double-hull tanker that reduces or eliminates many of the problems discussed above while at the same time meeting or exceeding the minimum requirements for double-hull tankers in current U.S. and international regulations.

SUMMARY OF THE INVENTION

[0023] The present invention is made to overcome the foregoing problems.

[0024] An object of the present invention is to provide a double-hull tanker meeting the requirements of existing United States and international regulations for the construction of double-hull tankers, having ballast tanks that are free of many of the problems discussed above.

[0025] It is a further object of the present invention to provide a double-hull tanker with ballast tanks having sufficient capacity but also having a reduced amount of surface area from the amount of ballast tank surface area in conventional double-hull tanker design. Further, the surface area of the boundary between ballast tanks and cargo tanks is also reduced, this boundary being particularly susceptible to coating failures.

[0026] It is another object of the present invention to provide a double-hull tanker having ballast tanks which are easier to construct than ballast tanks in conventional double-hull tankers.

[0027] Another object of the present invention is to provide a double-hull tanker having ballast tanks which are less susceptible to corrosion and less difficult, dangerous and expensive to inspect, maintain and repair than ballast tanks in conventional double-hull tankers.

[0028] It is still a further object of the present invention to provide a double-hull tanker having an accessible void in the double bottom which is not used for ballast, which remains dry during operation and, which as a result, is less susceptible to corrosion and is easy to inspect, maintain and repair.

[0029] Another objective of the present invention is to reduce free surface effects during ballasting operations by making the double bottom space a void.

[0030] Another object of the present invention is to provide a double-hull tanker having ballast tanks located near to or above the ballast water line, thus minimizing the need for piping in the ballast tanks.

[0031] Another object of the present invention is to reduce silt retention and facilitate tank cleaning in the ballast tanks by providing a smooth bottom surface in the ballast tanks.

[0032] To accomplish the above objectives, there is provided a tanker for carrying petroleum products, including crude oil, chemicals or other potentially environmentally harmful materials, having an inner hull and an outer hull. In the preferred embodiment, the void between the inner hull and outer hull at the bottom of the tanker is at or near the minimum spacing required by regulations. The tanker has side tanks extending, in the preferred embodiment, from near to or above the ballast water line. These side tanks serve to hold the ballast. The dimensions of the side tanks must be large enough to hold a sufficient volume of ballast water to keep the tanker in ballast condition meeting U.S. and international regulations for ballast draft. The cross-sectional shape of the side tank then is substantially rectangular in the preferred embodiment, although other shapes to most efficiently utilize the space available on the tanker ship also can be used. The tanker also has one or more cargo tanks located inside the inner hull, which tanks also may be substantially rectangular in cross-section and arranged to meet U.S. and international regulations for oil outflow.

[0033] The space in the bottom of the tanker, between the outer hull and the inner hull, beneath the cargo tank bottom and the ballast tanks, is left empty. It is dimensioned to permit access for inspection and maintenance. In this tanker arrangement, this space is not used for either ballast or oil or other liquid cargo, thereby remaining dry and accessible for inspection purposes. Furthermore, because it remains dry, lighting can be included as well as walkways or other components to facilitate inspection and maintenance.

[0034] The present invention provides an improved general arrangement for ballast tanks and cargo tanks in double-hull tankers. The double bottom required by regulation is assigned as inert accessible voids, thereby minimizing routine maintenance and repair of these spaces. All required ballast is stowed in the wide side tanks located near to or above the ballast water line in the preferred embodiment. This allows further simplification of the ballast operations through gravity discharge and minimization of ballast piping in tanks. The ballast tanks can be emptied by simply opening a valve strategically placed in the ballast tank which will allow the water in the tanks to flow out freely. Ballast tank stripping may or may not be incorporated. These design changes require greater hull depth to provide sufficient ballast and cargo volumes; however, for some tanker sizes, the centerline bulkhead may be eliminated, resulting in minimal net increase in hull steel weight. This is possible because, in the preferred embodiment, there is no free surface effect in the double bottom (because it is an empty void) when simultaneously discharging cargo and loading ballast.

[0035] Coating area in the ballast tanks that is exposed to salt-water is also substantially reduced in the preferred embodiment through adoption of shorter and wider tanks. The coating area on the boundary between the ballast tanks and heated cargo tanks is also substantially reduced in the preferred embodiment.

[0036] Ship structure is also simplified in the preferred embodiment of the present invention, owing to the relatively rectangular shape of the ballast tanks. The result is the reduction in unique, transitional details as well as the elimination of horizontal or diagonal knuckles, or joints, created where two steel plates intersect at an oblique angle. This joint is prone to cracking and has an increased chance of being affected by corrosion.

[0037] Also, in the preferred embodiment, the shape of the hull is fully accommodated without the need for a hopper, the sloped intersection of the inner bottom and inner side. In a conventional double-hull, the ballast tank must conform to the shape of the outer hull as the two sides of the outer hull come together at the bow, or front of the ship, in order to meet the minimum double-hull spacing. In order to conform with the outer hull and maintain regulatory compliance for inner and outer hull separation, the sides of the ballast tank must be sloped. This is eliminated in the preferred embodiment, where the ballast tanks are of rectangular shape and thus, easier to construct than the hopper found in conventional double-hull tankers.

[0038] The high ballast double-hull tanker will also realize other benefits, including reduced fatigue to the ship and crew through reduced vessel motions and accelerations in both loaded and ballasted conditions. Additionally, the high ballast tanker will have a greater reserve buoyancy from the deeper hull and higher margin line, resulting in safer operations in the event of a collision.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The features of the present invention are explained in more detail with reference to the following drawings, in which like reference numerals refer to like elements and in which:

[0040]FIG. 1 is a side view of a prior art conventional double-hull tanker;

[0041]FIG. 2 is a top view of the prior art conventional double-hull tanker of FIG. 1;

[0042]FIG. 3 is a view of the cross-section A-A of the prior art conventional double-hull tanker of FIGS. 1 and 2;

[0043]FIG. 4 is a detailed schematic of the structural layout of the cargo and ballast tank of the prior art conventional double-hull tanker of FIG. 3;

[0044]FIG. 5 is a side view of a tanker with the design of one embodiment of the present invention;

[0045]FIG. 6 is a top view of the tanker of FIG. 5; and

[0046]FIG. 7 is a view of the cross-section B-B of the tanker of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0047] The present invention will be explained with reference to the above mentioned figures. To more fully understand the present invention, it may be helpful to review the prior art of tankers.

[0048] Referring now to FIGS. 1-3, there is shown a conventional double-hull tanker 10, having an outer side hull 12 and an inner side hull 14. The conventional double-hull tanker 10 also has an outer bottom hull 16 and an inner bottom hull 18, commonly referred to as a doublebottom. One or more cargo tanks 20 are contained within the inner side hull 14 and the inner bottom hull 18. A plurality of ballast tanks 22 are contained within the space between the inner side hull 14 and outer side hull 12 and between the inner bottom hull 18 and outer bottom hull 16. Bulkheads 24 separate the space created within the inner side hull 16 and inner bottom hull 18 into compartments, resulting in the cargo tanks 20. Longitudinal bulkheads 26 and 28 may be installed to create two or more cargo tanks 20 and ballast tanks 22, respectively, across the width of the ship. The longitudinal bulkheads 26 and 28 are required to rectify the problem presented when the ballast tank and the cargo tank are simultaneously loaded as discussed above or as needed to provide adequate structural strength in very large tankers. Sloped surface 30, the hopper, is required throughout the length of the tanker due to the structural transitions that take place at the ends of the tanker body. In this conventional arrangement, the space between the inner hull (14 and 18) and the outer hull (12 and 16) is configured to load saltwater ballast. The proportions of these ballast spaces, tanks in their entirety, are such that the longest dimension in cross section is over 10 times the width of space separating the inner and outer hulls.

[0049]FIG. 4 shows a more detailed cross section of a cargo tank 20 in a conventional double-hull tanker and the structural requirements for creating the double-hull which is used for ballast tanks 22.

[0050] Now referring to FIGS. 5-7, showing one embodiment of the present invention, there is shown a side view, top view and a cross section. There consists a high ballast double-hull tanker 110 having an outer side hull 112 and an inner side hull 114. The high ballast also has an outer bottom hull 116 and an inner bottom cargo hull 118 as well as an inner bottom ballast hull 132. One or more cargo tanks 120 are contained within the inner side hull 114 and the inner bottom hull 118. Bulkheads 124 separate the space created within the inner side hull 116 and inner bottom cargo hull 118, into compartments, resulting in the cargo tanks 120. The number of cargo tanks can vary, depending on the length and desired capacity of the tanker and is not limited by the number shown in FIGS. 5-7. Longitudinal bulkhead 126 may be installed to create two or more cargo tanks 120 across the width of the tanker. The proportions of the ballast tank are such that the longest dimension in cross section is a multiple less than or equal to five times the width between the inner and outer hull.

[0051] The ballast tanks 122 in the preferred embodiment of the present invention are located between the outer hull 112 and the inner hull 114 and extend from the top 134 of the tanker 110 to the bottom 132 of the ballast tank 122, which is at or near the ballast water line D. The ballast water line D is a calculated depth above the bottom hull 116 of the tanker 110 and is based upon the weight of the tanker and the volume of ballast to keep the tanker in ballast condition as specified by U.S. or international regulations. The space 136 between the outer bottom hull 116 and the inner bottom hull 118 and the bottom 132 of the ballast tanks 122 is an isolated accessible space which remains dry during all conditions of tanker operation.

[0052] In the preferred embodiment, the ballast tanks contain at least one valve 138 located at or above the bottom 132 of the ballast tank 122. In operation, the valve 138 is opened in order to drain the ballast water out of the ballast tanks 122 without the use of pumps. Stripping systems to remove residual ballast may or may not be fitted.

[0053] The present invention could also be used in floating stationary objects such as offshore oil drilling structures or similar facilities having a need for ballast tanks.

[0054] While the present invention has been described above with respect to one preferred embodiment thereof, it should of course be understood that the present invention should not be limited only to that embodiment, but various changes or modifications may be made without departure from the scope of the invention as defined by the appended claims. 

We claim:
 1. A tanker comprising: a ship body having sidewalls and a bottom, said sidewalls having a top and a bottom, said bottom of said sidewalls being attached to said bottom of said ship body; at least one cargo tank disposed within said ship body, said cargo tank having sidewalls and a bottom, said sidewalls having a top and a bottom, said bottom of said sidewalls being attached to said bottom of said cargo tank, said bottom of said cargo tank being disposed at a height higher than the bottom of said tanker ship body, thereby creating a first space between said bottom of said cargo tank and said bottom of said ship body; at least two ballast tanks, arranged on opposite sides of said cargo tank, between said cargo tank and said sidewalls of said ship body, said ballast tanks having sidewalls and a bottom, said sidewalls of said ballast tanks having a top and a bottom, said bottom of said sidewalls of said ballast tanks being attached to said bottom of said ballast tanks, said bottom of said ballast tanks being disposed at a height higher than the bottom of said tanker ship body, thereby creating a second space between said bottom of said ballast tanks and said bottom of said ship body.
 2. The tanker of claim 1, wherein said cargo tanks are centrally located along the longitudinal axis of said ship body.
 3. The tanker of claim 1, wherein the bottoms of said ballast tanks are disposed at a height at or near the ballast water line for said tanker.
 4. The tanker of claim 1, wherein the first space is in fluid communication with the second space.
 5. The tanker of claim 1, wherein said ballast tanks further include at least one valve having an open position and a closed position, whereby when said valve is in said open position, ballast water will empty from said ballast tanks under the force of gravity.
 6. The tanker of claim 1, wherein said ballast tanks are proportioned such that the longest dimension in cross section is no greater than five times the shortest dimension of said ballast tanks, regardless of shape.
 7. A ship comprising: a ship body having sidewalls and a bottom portion, said sidewalls having a top and a bottom, said bottoms of said sidewalls attached to said bottom portion; at least one cargo tank disposed within said ship body, said cargo tank arranged such that a first space is created between said cargo tank and said bottom portion of said ship body; at least two ballast tanks, arranged on opposite sides of said cargo tank between said cargo tank and said sidewalls of said ship body, said ballast tanks arranged such that a second space is created between said ballast tanks and said bottom portion of said ship body.
 8. A tanker ship for carrying petroleum products, chemicals or other liquids harmful to the environment, the tanker ship comprising: an outer hull having a bottom; at least one cargo tank disposed inside said outer hull; two ballast tanks for each cargo tank, the ballast tanks being disposed inside said outer hull and on opposite sides of said cargo tank between said cargo tank and said outer hull; a first void between said cargo tank and the bottom of said outer hull; a second void between one of said ballast tanks and the bottom of said outer hull; and a third void between the other of said ballast tanks and the bottom of said outer hull.
 9. The tanker ship of claim 8, wherein the first void is in fluid communication with the second void and the third void.
 10. A tanker ship comprising: an outer hull having a bottom; at least one cargo tank dispersed inside said outer hull; at least one ballast tank having a first portion and a second portion, the first portion located on a first side of said cargo tank and the second portion located on a second side of said cargo tank, the first portion being in fluid communication with the second portion; a first void between said cargo tank and the bottom of said outer hull; a second void between the first portion of said ballast tank and the bottom of said outer hull; and a third void between the second portion of said ballast tank and the bottom of said outer hull.
 11. The tanker ship of claim 10 wherein the second void is in fluid communication with the third void.
 12. The tanker ship of claim 10 wherein the first, second and third voids are in fluid communication with each other.
 13. A vessel for floating on water comprising: a structure for floating on water and adapted for being loaded and unloaded; and at least two ballast tanks adapted for containing ballast water when the vessel is not carrying a full load, said ballast tanks being mounted on said structure at a height such that the ballast water may be drained by gravity.
 14. The vessel of claim 13 wherein the structure for floating on water has a double hull.
 15. A vessel for floating on water while either carrying a full load or not carrying a full load, comprising: a structure for floating on water and adapted for being loaded and unloaded; and at least two ballast tanks adapted for containing ballast water when the vessel is not carrying a full load, said ballast tanks being mounted on said structure such that the ballast water is contained at an elevation above the water line of the vessel.
 16. The vessel of claim 15 wherein the structure for floating on water has a double hull. 